Salicylic acid is not required for Cf-2- and Cf-9-dependent resistance of tomato to Cladosporium fulvum

Tomato leaves or cotyledons expressing the Cf-2 or Cf-9 Cladosporium fulvum resistance genes induce salicylic acid (SA) synthesis following infiltrati on with intercellular washing fluid (IF) containing the fungal peptide elic itors Avr2 and Avr9. We investigated whether SA was required for Cf gene-de pendent resistance. Tomato plants expressing the bacterial gene nahG, encod ing salicylate hydroxylase, did not accumulate SA in response to IF infiltr ation but remained fully resistant to C. fulvum. NahG Cf0 plants were as su sceptible to C. fulvum as wild-type Cf0. Neither free nor conjugated salicy lic acid accumulated in IF-infiltrated Cf2 and Cf9 NahG leaves and cotyledo ns but conjugated catechol did accumulate. The Cf9-dependent necrotic respo nse to IF was prevented in NahG plants and replaced by a chlorotic Cf-2-lik e response. SA also potentiated Cf-9-mediated necrosis in IF-infiltrated wi ld-type leaves. In contrast, the Cf-2-dependent IF response was retained in NahG leaves and chlorosis was more pronounced than in the wild-type. The d istribution of cell death between different cell types was altered in both Cf2 and Cf9 NahG leaves after IF injection. IF-induced accumulation of thre e SA-inducible defence-related genes was delayed and reduced but not abolis hed in NahG Cf2 and Cf9 leaves and cotyledons. NahG Tm-2(2) tomato showed i ncreased hypersensitive response (HR) lesion size upon TMV infection, as ob served in TMV-inoculated N gene-containing NahG tobacco plants.